The present invention relates to a thin film electroluminescence device including a luminescence emitting layer comprising an alkaline-earth selenide, such as SrSe (strontium selenide) and calcium selenide (CaSe), or an alkaline-earth sulfide, such as CaS (calcium sulfide) and strontium sulfide (SrS), as the luminescent host material, and a lanthanoid rare-earth element as an activator, and a method of fabricating the thin film electroluminescence device.
Zinc sulfide (ZnS) which is well known as a host material for an electroluminescence device can attain orange electroluminescent (hereinafter referred to as EL) emission with addition thereto of manganese as an activator, and green EL emission with addition thereto of terubium as an activator. However, blue EL emission cannot be attained by use of zinc sulfide. Therefore, it cannot be used as a host material for a multi-color EL device.
In contrast to this, it is conventionally known that calcium sulfide (CaS) and strontium sulfide (SrS) can be used as a host material for a multi-color EL device when a lanthanoid rare-earth element is used as an activator for them, as reported, for example, in W. Lehmann, J. Lumin. 5, 87(1972), and A. Vecht and J. Mayo, SID (Society for Information Displays) International Symposium Digest of Technical Papers, p. 88 to 89 (1977).
Alkaline-earth selenides, such as SrSe and CaSe, however, have not been studied as a host material for EL devices. This is considered to be not because such selenides have some intrinsic defects in the crystalline structure, but because it has been extremely difficult to prepare those selenides with high purity in a flawless manner.
Alkaline-earth sulfides, such as CaS (calcium sulfide) and strontium sulfide (SrS), belong to the group of IIa-VIb compounds and have melting points as high as about 2000.degree. C. Further, the alkaline-earth metals themselves are so easily oxidized that when producing a thin film of such alkaline-earth sulfides, the electron beam (EB) evaporation method and the sputtering method have to be employed. The thin films of the alkaline-earth sulfides prepared by the above methods, however, have the following shortcomings:
(1) It is extremely difficult to purify SrS and CaS to the extent that the purified compounds satisfy their respective stoichiometric compositions.
(2) Extremely difficult treating processes are required for the formation of a thin film of such alkaline-earth sulfides as described in a report by V. Shanker et al., at page 509 of the 31 Applied Physics Session Hand-out (1984. 3). Specifically, coevaporation of CaS and S, CaS by the EB evaporation and S by the resistance heating evaporation, must be performed to deposit a film on a substrate, and after the formation of a thin film, the film must be thermally treated in vacuum or in an atmosphere of hydrogen sulfide (H.sub.2 S) at a temperature which is much higher than the temperature of the substrate at the film deposition. If such after-treatment is not performed, EL characteristics necessary for practical use cannot be obtained.
This is considered to be because sulfur has an extremely higher vapor pressure and is much more easily sublimed as compared with calcium (Ca), strontium (Sr), and numerous S-vacant points are formed in the film, so that the composition of the film becomes non-uniform in the direction of the thickness of the film. These problems stem from the properties of the materials themselves and cannot be solved by the EB evaporation method. (3) The EB evaporation method is not suitable for the fabrication of EL devices, because it cannot always yield a film having a uniform thickness. In other words, the films prepared by the EB evaporation method have a relatively large distribution in terms of the thickness thereof. Furthermore, a large film having a uniform thickness is necessary for EL devices and the performance of EL devices largely depend upon the uniformity of the thickness of the film.